Pichia is a methylotrophic yeast that is widely used for the production of heterologous proteins of industrial and academic interest (Cregg, 1998; Higgins and Cregg, 1998). FLD is an important enzyme in the utilization of methanol as a carbon and energy source (Veenhuis et al., 1983). In methylotrophic yeasts, the methanol metabolic pathway is thought to be nearly the same, beginning with the oxidation of methanol to formaldehyde by alcohol oxidase (AOX), a hydrogen peroxide-producing oxidase that is sequestered in an organelle called the peroxisome. Hydrogen peroxide is then degraded to oxygen and water by catalase, the classic peroxisomal marker enzyme. A portion of the resulting formaldehyde condenses with xylulose-5′-monophosphate in a reaction catalyzed by dihydroxyacetone synthase (DAS), the third peroxisomal methanol pathway enzyme. The products of this reaction, glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone, then leave the peroxisome and enter a cyclic pathway that regenerates xylulose-5′-monophosphate and also generates one net molecule of GAP for every three turns of the cycle. GAP is used for biosynthesis of carbon skeletons for cell growth. Another portion of the formaldehyde leaves the peroxisome and is oxidized to formate by formaldehyde dehydrogenase (FLD) and then to carbon dioxide by formate dehydrogenase (FDH). Both of these reactions produce reducing power in the form of NADH. One model of FLD function is that the NADH generated by FLD and FDH serves as the primary source of energy during growth on methanol (Veenhuis et al., 1983). The second model proposes that most energy for methanol growth comes from the oxidation of one or more of the xylulose-5′-monophosphate cycle intermediates by tricarboxcylic acid cycle enzymes, and that the primary role of FLD is to protect the cell from toxic formaldehyde that accumulates with excess methanol in the medium (Sibirny et al., 1990).
In addition to methanol, FLD is also involved in the metabolism of certain methylated amines (e.g. methylamine and choline) as sole nitrogen sources (Zwart et al., 1980). In this pathway, amine groups are first liberated by a peroxisomal amine oxidase, leaving formaldehyde which is further oxidized by FLD and FDH. When growing on methylamine as sole nitrogen source, high levels of FLD are induced even in the presence of excess glucose. Thus, the primary role of FLD in methylamine metabolism appears to be for protecting cells from the toxic effects of formaldehyde and not for generating carbon or energy.
FLD synthesis is regulated independently in response to either methanol as sole carbon source and energy source or to methylamine as sole nitrogen source. Thus, for example, only low levels of FLD are observed in cells growing on glucose- and ammonium ion-containing medium, whereas on either methanol-ammonium ion or glucose-methylamine media, FLD levels are high.
In the Pichia system, most foreign genes are expressed under the transcriptional control of the P. pastoris alcohol oxidase 1 gene promoter (PAOX1), the regulatory characteristics of which are well suited for this purpose. The promoter is tightly repressed during growth of the yeast on most common carbon sources, such as glucose, glycerol, or ethanol, but is highly induced during growth on methanol (Tschopp et al., 1987; U.S. Pat. No. 4,855,231 to Stroman, D. W., et al). For production of foreign proteins, PAOX1-controlled expression strains are initially grown on a repressing carbon source to generate biomass and then shifted to methanol as the sole carbon and energy source to induce expression of the foreign gene. One advantage of the PAOX1 regulatory system is that P. pastoris strains transformed with foreign genes whose expression products are toxic to the cells can be maintained by growing under repressing conditions.
Although many proteins have been successfully produced using PAOX1, this promoter is not appropriate or convenient in all settings. For example, in shake-flask cultures, methanol rapidly evaporates, and it is inconvenient to monitor methanol concentrations and repeatedly add the compound to the medium. In addition, the storage of large amounts of methanol needed for the growth and induction of PAOX1-controlled expression strains in large-volume high-density fermentor cultures is a potential fire hazard. There is a need therefore, for an alternative promoter to PAOX1, which is both transcriptionally efficient and regulatable by a less volatile and flammable inducer. The present invention provides the P. pastoris and Hansenula polymorpha formaldehyde dehydrogenase gene (FLD) promoter having both properties.
In addition, there is a need for a selectable marker which functions in methylotrophic yeasts other than a selectable marker which is an antibiotic resistance gene. At present, only the ZeoR gene can be used to transform into P. pastoris strains independent of their genotype. In addition, ZeoR is the only that gene can be used to directly select for P. pastoris strains that receive multiple copies of an expression vector (by increasing the concentration of Zeocin in selective medium). A second gene which confers resistance to the antibiotic G418 (G418R) can be used to screen for multicopy expression strains of P. pastoris but its use requires that an auxotrophic/biosynthetic gene selection marker must also be included in vectors to select for transformants. The FLD structural gene of the present invention may be used as a selectable marker in methylotrophic yeast cells and does not confer resistance to antibiotics.